It's unfair that such a burden is rested upon her back. How did she receive such a talent/destiny? And is there some sort of perk/bonus/boon/ability that she receives in return for absorbing other ponies' nightmares?

She obtainted her CM because of Luna. She met her during her dream and decided to try to meet her again by going into trance, as It would be more stable than dream, and longer as well. However she didn't get into normal dream, only to a nightmare of contemporary. After seeing His scare and suffering she helped him by absorbing bad dream, and changing it into good one. Unfortunately - because of doing that she lost her sight.It doesn't pay off to her physically, but mentally- It does, as she feels that she is going something good for others, and makes Their lifes easier :3